In producing aromatic hydrocarbons compounds having multi-substituents, mixed products in which various isomers are simultaneously present are generally obtained due to the limit of reaction process and reaction thermodynamical equilibrium. Said mixed products are necessary to be further separated to obtain the most valuable isomers for application. However, these isomers usually have very close boiling points such that it is difficult to separate them using conventional rectification process. In this case, selective adsorption is widely used in industry to achieve the separation of these isomers.
The principle of adsorptive separation technique is that an object product is separated from mixed materials and purified by certain adsorbents with characteristics of preferentially adsorbing or preferentially non-adsorbing the object product, in combination with suitable processes. Adsorbent is the basis and core of adsorptive separation technique. Now, it is known that X zeolite, ion exchanged with barium cation or potassium cation, alone or in combination, has a characteristic of preferentially adsorbing para-aromatic hydrocarbons isomers. BaX or BaKX thus is used widely as adsorbents in industry to adsorb p-xylene so as to separate it from C8 aromatic hydrocarbons isomers, in combination with continuous countercurrent simulation moving bed. P-xylene is obtained in high purity as following: p-xylene is adsorbed by an adsorbent in adsorbing column through repeating countercurrent mass transfer exchange utilizing the adsorbent's characteristic of preferentially adsorbing p-xylene, so that its concentration increases gradually; the adsorbed p-xylene is desorbed with a desorbent after desired product purity is achieved; and extracting solution is rectified to recover the desorbent. P-xylene produced by the process has a purity of up to 99.8 mass % and the yield is up to 98 mass %. In addition, U.S. Pat. No. 4,940,548 and U.S. Pat. No. 5,149,887 disclose that such adsorbents are used for separating isomers of diethyltoluene, methylphenol and the like.
An excellent adsorbent should have three properties, including high adsorption capacity, good selective adsorption ability and faster mass transfer speed. It is obvious that adsorption capacity of the adsorbent is proportional to the content of zeolite herein, that is, the higher is the content of zeolite, the greater is the adsorption capacity of the adsorbent. Since synthesized zeolite is usually in form of powder, it is necessary to agglomerate it by adding certain amount of binder in order to meet the requirement in industry, thereby resulting in partial loss of adsorption capacity. Therefore, reducing the amount of inert binder used in the adsorbent and converting it into zeolite as much as possible is an effective way to improve adsorption performance. U.S. Pat. No. 3,960,774 firstly discloses that an adsorbent precursor containing X or Y zeolite and binder is treated with an aqueous sodium hydroxide solution in order to improve the crystallinity of the adsorbent, and then is ionexchanged with barium or potassium cations.
The selectivity of the adsorbent is improved mostly in respect of the species of exchanged ions and the property of the zeolite. In U.S. Pat. No. 3,997,620, SrBaX adsorbent is prepared from exchanging with strontium and barium bi-metal ions to improve the selectivity for para-xylene. In U.S. Pat. No. 4,283,587, ionexchanged X or Y zeolite is treated with alkyl amine or alkyl ammonium hydrochloride to improve the selectivity for para-isomers. CN1275926A discloses an agglomerated zeolite adsorbent, wherein X zeolite having a Si/Al atomic ratio of 1-1.15 and low content of silicon dioxide is used as raw material to prepare adsorbent, and is exchanged with barium and potassium ions. Exchangeable sites of the adsorbent are at least 70% occupied by barium ions, and up to 30% occupied by potassium ions. Kaolin is used as the binder of said adsorbent, and this adsorbent is treated with an alkaline liquor in order to be crystallizated into X zeolite in-situ, thereby enhancing the capability of the adsorbent.
To improve the performance of the adsorbent, the mass transfer rate of the adsorbent should be improved besides improving adsorption capability and selectivity of the adsorbent. In CN1448213A and CN1565718A, X zeolites of small crystal grains with the crystal particulate size of 0.5-1.0 micron or 0.1-0.4 micron are used as active components of adsorbents in order to improve the mass transfer rate in crystallines of zeolites of the adsorbents.
CN1358566A discloses an adsorbent and its preparation process wherein the performance of the adsorbent is improved by improving the secondary pore distribution of the adsorbent. X zeolite or Y zeolite is mixed with binder, and 0.5-6.0 mass % of pore-expanding agent is added into the resulted mixture, following by mixing homogeneously, moulding by adding water, drying, activating, alkali treating and ion exchanging, to obtain an adsorbent. Said binder is one or more selected from kaolin, bentonite, bentone, silica sol, aluminium sol and water glass. Said pore-expanding agent is one or more selected from lignin, sodium cellulose and sesbania powder.